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Section 9.3 Summary – pages Cellular Respiration The process by which mitochondria break down food molecules to produce ATP is called cellular respiration. The process by which mitochondria break down food molecules to produce ATP is called cellular respiration. There are three stages of cellular respiration: glycolysis, the citric acid cycle, and the electron transport chain. There are three stages of cellular respiration: glycolysis, the citric acid cycle, and the electron transport chain.

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Cellular Respiration Respiration occurs in three metabolic stages: glycolysis, the Krebs cycle, and the electron transport chain and oxidative phosphorylation. Respiration occurs in three metabolic stages: glycolysis, the Krebs cycle, and the electron transport chain and oxidative phosphorylation.

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Section 9.3 Summary – pages Cellular Respiration The first stage, glycolysis, is anaerobic—no oxygen is required. The first stage, glycolysis, is anaerobic—no oxygen is required. The last two stages are aerobic and require oxygen to be completed. The last two stages are aerobic and require oxygen to be completed.

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Section 9.3 Summary – pages Glycolysis Glycolysis is a series of chemical reactions in the cytoplasm of a cell that break down glucose, a six- carbon compound, into two molecules of pyruvic acid, a three-carbon compound. Glycolysis is a series of chemical reactions in the cytoplasm of a cell that break down glucose, a six- carbon compound, into two molecules of pyruvic acid, a three-carbon compound. Glucose 2ATP 2ADP 2PGAL 4ADP + 4P 2NAD+ 2NADH + 2H + 4ATP 2 Pyruvic acid

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Section 9.3 Summary – pages Before citric acid cycle and electron transport chain can begin, pyruvic acid undergoes a series of reactions in which it gives off a molecule of CO 2 and combines with a molecule called coenzyme A to form acetyl-CoA. Before citric acid cycle and electron transport chain can begin, pyruvic acid undergoes a series of reactions in which it gives off a molecule of CO 2 and combines with a molecule called coenzyme A to form acetyl-CoA. Pyruvic acid Outside the mitochondrion Mitochondrial membrane Inside the mitochondrion Pyruvic acid Intermediate by-product NAD + NADH + H + CO 2 Coenzyme A - CoA Acetyl-CoA Glycolysis

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Section 9.3 Summary – pages Fermentation During heavy exercise, when your cells are without oxygen for a short period of time, an anaerobic process called fermentation follows glycolysis and provides a means to continue producing ATP until oxygen is available again. During heavy exercise, when your cells are without oxygen for a short period of time, an anaerobic process called fermentation follows glycolysis and provides a means to continue producing ATP until oxygen is available again.

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Section 9.3 Summary – pages Alcoholic fermentation Another type of fermentation, alcoholic fermentation, is used by yeast cells and some bacteria to produce CO 2 and ethyl alcohol. Another type of fermentation, alcoholic fermentation, is used by yeast cells and some bacteria to produce CO 2 and ethyl alcohol.

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Section 9.3 Summary – pages Lactic acid fermentation Lactic acid fermentation is one of the processes that supplies energy when oxygen is scarce. Lactic acid fermentation is one of the processes that supplies energy when oxygen is scarce. In this process, the reactions that produced pyruvic acid are reversed. In this process, the reactions that produced pyruvic acid are reversed. Two molecules of pyruvic acid use NADH to form two molecules of lactic acid. Two molecules of pyruvic acid use NADH to form two molecules of lactic acid.

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Section 9.3 Summary – pages Lactic acid fermentation This releases NAD + to be used in glycolysis, allowing two ATP molecules to be formed for each glucose molecule. This releases NAD + to be used in glycolysis, allowing two ATP molecules to be formed for each glucose molecule. The lactic acid is transferred from muscle cells, to the liver that converts it back to pyruvic acid. The lactic acid is transferred from muscle cells, to the liver that converts it back to pyruvic acid.

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Section 9.3 Summary – pages The citric acid cycle The citric acid cycle, also called the Krebs cycle, is a series of chemical reactions similar to the Calvin cycle in that the molecule used in the first reaction is also one of the end products. The citric acid cycle, also called the Krebs cycle, is a series of chemical reactions similar to the Calvin cycle in that the molecule used in the first reaction is also one of the end products. For every turn of the cycle, one molecule of ATP and two molecules of carbon dioxide are produced. For every turn of the cycle, one molecule of ATP and two molecules of carbon dioxide are produced.

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Section 9.3 Summary – pages Formation of CO 2 A molecule of CO 2 is formed, reducing the eventual product to a five- carbon compound. In the process, a molecule of NADH and H + is produced. Formation of CO 2 A molecule of CO 2 is formed, reducing the eventual product to a five- carbon compound. In the process, a molecule of NADH and H + is produced. NAD + NADH + H + O==O (CO 2 ) The citric acid cycle

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Section 9.3 Summary – pages Formation of the second CO 2 Another molecule of CO 2 is released, forming a four- carbon compound. One molecule of ATP and a molecule of NADH are also produced. Formation of the second CO 2 Another molecule of CO 2 is released, forming a four- carbon compound. One molecule of ATP and a molecule of NADH are also produced. NAD + NADH + H + O= =O (CO 2 ) ADP + ATP The citric acid cycle

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FADH 2 NADH + H + Section 9.3 Summary – pages Recycling of oxaloacetic acid The four-carbon molecule goes through a series of reactions in which FADH 2, NADH, and H + are formed. The carbon chain is rearranged, and oxaloacetic acid is again made available for the cycle. Recycling of oxaloacetic acid The four-carbon molecule goes through a series of reactions in which FADH 2, NADH, and H + are formed. The carbon chain is rearranged, and oxaloacetic acid is again made available for the cycle. NAD + FAD The citric acid cycle

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Only 4 of 38 ATP ultimately produced by respiration of glucose are derived from substrate-level phosphorylation. Only 4 of 38 ATP ultimately produced by respiration of glucose are derived from substrate-level phosphorylation. The vast majority of the ATP comes from the energy in the electrons carried by NADH (and FADH 2 ). The vast majority of the ATP comes from the energy in the electrons carried by NADH (and FADH 2 ). The energy in these electrons is used in the electron transport system to power ATP synthesis. The energy in these electrons is used in the electron transport system to power ATP synthesis. The inner mitochondrial membrane couples electron transport to ATP synthesis: a closer look The inner mitochondrial membrane couples electron transport to ATP synthesis: a closer look

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Electrons from NADH or FADH 2 ultimately pass to oxygen. Electrons from NADH or FADH 2 ultimately pass to oxygen. –For every two electron carriers (four electrons), one O 2 molecule is reduced to two molecules of water. The electron transport chain generates no ATP directly. The electron transport chain generates no ATP directly. Its function is to break the large free energy drop from food to oxygen into a series of smaller steps that release energy in manageable amounts. Its function is to break the large free energy drop from food to oxygen into a series of smaller steps that release energy in manageable amounts. The movement of electrons along the electron transport chain does contribute to chemiosmosis and ATP synthesis. The movement of electrons along the electron transport chain does contribute to chemiosmosis and ATP synthesis.

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During respiration, most energy flows from glucose -> NADH -> electron transport chain -> proton-motive force -> ATP. During respiration, most energy flows from glucose -> NADH -> electron transport chain -> proton-motive force -> ATP. Considering the fate of carbon, one six- carbon glucose molecule is oxidized to six CO 2 molecules. Considering the fate of carbon, one six- carbon glucose molecule is oxidized to six CO 2 molecules. Some ATP is produced by substrate-level phosphorylation during glycolysis and the Krebs cycle, but most comes from oxidative phosphorylation. Some ATP is produced by substrate-level phosphorylation during glycolysis and the Krebs cycle, but most comes from oxidative phosphorylation. Cellular respiration generates many ATP molecules for each sugar molecule it oxidizes: a review Cellular respiration generates many ATP molecules for each sugar molecule it oxidizes: a review

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Each NADH from the Krebs cycle and the conversion of pyruvate contributes enough energy to generate a maximum of 3 ATP (rounding up). Each NADH from the Krebs cycle and the conversion of pyruvate contributes enough energy to generate a maximum of 3 ATP (rounding up). –The NADH from glycolysis may also yield 3 ATP. Each FADH 2 from the Krebs cycle can be used to generate about 2ATP. Each FADH 2 from the Krebs cycle can be used to generate about 2ATP. In some eukaryotic cells, NADH produced in the cytosol by glycolysis may be worth only 2 ATP. In some eukaryotic cells, NADH produced in the cytosol by glycolysis may be worth only 2 ATP. –The electrons must be shuttled to the mitochondrion. –In some shuttle systems, the electrons are passed to NAD +, in others the electrons are passed to FAD.

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How efficient is respiration in generating ATP? How efficient is respiration in generating ATP? –Complete oxidation of glucose releases 686 kcal per mole. –Formation of each ATP requires at least 7.3 kcal/mole. –Efficiency of respiration is 7.3 kcal/mole x 38 ATP/glucose/686 kcal/mole glucose = 40%. –The other approximately 60% is lost as heat. Cellular respiration is remarkably efficient in energy conversion. Cellular respiration is remarkably efficient in energy conversion.

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Section 9.3 Summary – pages Comparing Photosynthesis and Cellular Respiration Photosynthesis Cellular Respiration Food synthesized Food broken down Energy from sun stored in glucose Energy of glucose released Carbon dioxide taken in Carbon dioxide given off Oxygen given off Oxygen taken in Produces sugars from PGALProduces CO2 and H 2 O Requires light Does not require light Occurs only in presence of chlorophyll Occurs in all living cells Table 9.1 Comparison of Photosynthesis and Cellular Respiration